Method for depositing and etching ruthenium layers

ABSTRACT

The present invention provides a method for purifying ruthenium sources to obtain high purity ruthenium metal and form a ruthenium metal pattern on a semiconductor substrate without the need for high temperature processing or a complex series of wet processes. A gas stream including ozone (O 3 ) is brought into contact with a ruthenium source in one or more reaction vessels to form ruthenium tetraoxide (RuO 4 ), a compound that is a gas at the reaction conditions. The ruthenium tetraoxide, along with unreacted ozone and the remainder of the gas stream is then fed into a collection vessel where the gaseous ruthenium tetraoxide is reduced to form a ruthenium dioxide (RuO 2 ) layer on a semiconductor substrate. The deposited ruthenium dioxide is then reduced, preferably with hydrogen, to produce highly pure ruthenium metal that may be, in turn, patterned and dry etched using ozone as an etchant gas.

PRIORITY STATEMENT

This application is a divisional application from U.S. patentapplication Ser. No. 10/170,686 filed Jun. 14, 2002, now abandoned,which was a divisional application from U.S. patent application Ser. No.09/655,307 filed Sep. 5, 2000, now U.S. Pat. No. 6,458,183 B1, which isincorporated herein, in its entirety, by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for theproduction of highly pure ruthenium and ruthenium dioxide.

2. Description of the Related Art and General Background

A member of the platinum group, ruthenium occurs naturally with othermembers of the platinum group in ores found in Russia's Ural mountains,North and South America, and particularly, South Africa. It is alsofound along with other platinum metals in small but commercialquantities in both the pentlandite of the Sudbury, Ontario,nickel-mining region, and in the pyroxinite deposits of South Africa.Commercially, ruthenium may be isolated from the other platinum metalsthrough several complex chemical processes, the final stage of whichgenerally includes the hydrogen reduction of ammonium ruthenium chlorideor nitrosylruthenium chloride, to produce ruthenium metal powder.

Ruthenium, a hard, white metal, is one of the most effective hardenersfor platinum and palladium and is typically alloyed with these metals toproduce electrical contacts for severe wear resistance. There have alsobeen reports that a ruthenium alloy, specifically a ruthenium-molybdenumalloy, exhibits superconductivity at 10.6° K. It has also been reportedthat the corrosion resistance of titanium can be improved over 100 timesby adding as little as 0.1% ruthenium. Ruthenium is also a versatilecatalyst and is frequently used in petrochemical and other industrialprocesses to remove H₂S.

One method for extracting ruthenium is disclosed in U.S. Pat. No.3,997,337 (“the '337 patent”). The '337 patent included a discussion ofboth earlier methods for the separation and purification of preciousmetals, including ruthenium, from a concentrate of by-metals and theimproved method taught by the patent. The improvement disclosed in the'337 patent for the separation and purification of precious metals,including ruthenium, from a concentrate of by-metals comprised heatingthe concentrate to a temperature between 1100° C. and 1500° C.,preferably at about 1300° C., in a gaseous stream which comprisesoxygen. This heating step is continued for a period sufficient to ensurequantitative removal of one or more of lead, arsenic, silver, bismuthand/or tellurium and the oxidation of ruthenium, rhodium and iridium.The referenced by-metal concentrate is obtained as a by-product of theseparation of platinum, palladium, and gold from an ore or other mixedsource.

In the previous process, the by-metal concentrate was fused withpotassium bisulphate (KHSO₄) to convert the rhodium to the water-solublesulphate, Rh₂(SO₄)₃, which can be removed by washing. The remainingresidue was then subjected to a sodium peroxide (Na₂O₂) fusion toconvert the ruthenium and osmium to water soluble sodium salts of theiroxo-anions (e.g. RuO₄ ²⁻ and Os₄ ²⁻ respectively) and to convert theiridium to an acid soluble hydrous oxide (probably IrO₂.nH₂O). Theruthenium and osmium were then separated from the iridium by treatingthe sodium peroxide melt with water to form a precipitate, and treatingthe precipitate with hydrochloric acid to dissolve the iridium. Theruthenium and osmium were then normally purified by a collectivechlorine distillation, followed by a nitric acid distillation forosmium. The rhodium is treated for the removal of impurities such aspalladium, tellurium and other base metals that are also renderedsoluble by the KHSO₄ fusion. The iridium has to be separated from largequantities of lead and other impurities present in the concentrate whichhave been rendered soluble by the sodium peroxide (Na₂O₂) fusion. As canbe appreciated, this method used both large quantities of concentratesand correspondingly large quantities of costly reagents. Further, theimpurities, in particular tellurium were sometimes difficult to remove.

The improvement outlined in the '337 patent was intended to provide aprocess for the treatment of a by-metal concentrate for 1) to removetroublesome impurities such as Te, As, Bi, Ag, and Pb; 2) the removal ofosmium; and 3) to reduce the bulk of the by-metals being refined therebyproviding saving in both reagents and equipment. This was accomplishedby treating a concentrate of by-metals by heating to between about 1100°C. and 1500° C. in an oxygen-containing gaseous stream for a period oftime (examples include times of 20 hours) sufficient to ensure bothquantitative removal of one or more of lead, arsenic, silver, bismuthand/or tellurium and the oxidation of ruthenium, rhodium and iridium totheir oxides. According to the patent, the oxygen-containing gaseousstream could be air and the exhaust gas could be scrubbed with a liquidto recover osmium. The '337 patent also provided for the separation ofruthenium from the other platinum group metals by fusing the ignitedby-metal concentrate with potassium hydroxide and leaching the melt withwater to dissolve ruthenium complexes formed during the fusion process.As described in the '337 patent, a by-metal concentrate was heated toabout 1300° C. for 20 hours in a stream of air, a process by whichosmium, together with lead, arsenic, silver, bismuth and tellurium, werequantitatively removed from the concentrate while less than 10% of theruthenium and only traces of the other platinum group metals werevolatilized. The vapors were scrubbed with a 10% NaOH solution toprecipitate all the metals, with the exception of ruthenium and osmium,as hydrous oxides (which settle to the bottom of the receiving vessel).The ruthenium and osmium oxides which are converted to soluble sodiumsalts according to the following reactions:RuO₄+NaOHΠNa₂(RuO₄)+½O₂+H₂O  (a)OsO₄+2NaOHΠNa₂(OsO₄(OH)₂)  (b)

The ruthenium was then precipitated from the alkali solution by theaddition of ethanol to reduce the oxo-anion RuO₄ ²⁻ and precipitate theinsoluble hydrous oxide (reported as Ru₂O₃.nH₂O but the applicantsbelieve RuO₂.nH₂O may be more accurate). This precipitate is filteredoff together with the sludge in the receiver which contains the othermetals which have been volatized and is recycled to the lead alloyingstage of the metal process or to some other convenient point if leadalloying is not utilized. The osmium remaining in solution is thenprecipitated at room temperature as a hydrous oxide (reported asOS₂O₃.nH₂O, but the applicants believe OsO₂.nH₂O may be more accurate)by acidifying the solution with HCl to a pH of 4.0.

U.S. Pat. No. 4,105,442 (“the '442 patent”) teaches an alternativeprocess for the separation and purification of ruthenium involving theconversion of the ruthenium present in solution to a nitrosylrutheniumcomplex with the ruthenium in the Ru+2 oxidation state. Thenitrosylruthenium complex is then converted to a nitrosylrutheniumchlorocomplex, which is then removed from solution using a suitableliquid or resin anion exchanger.

The '442 patent notes the existence of conventional techniques for therecovery and purification of ruthenium and osmium based on the formationof low boiling temperature oxides in solution, with the oxides beingsubsequently removed from the solution by heating. For osmium, oxidationof the metal to the VIII oxidation state is relatively easy, and anumber of oxidizing agents can be used. Furthermore, osmium can beefficiently removed as the tetraoxide forms even under fairly stronglyacid conditions. In the case of ruthenium however, the oxidation is moredifficult and control of the solution pH at a relatively high value isessential. Under these circumstances, removal of ruthenium from solutionis incomplete, typically leaving several hundred parts per million ofruthenium in the solution. This not only represents a loss in rutheniumrecovery, but the remaining ruthenium constitutes an impurity elementduring the refining and recovery of the other platinum group metals.Further disadvantages of this process include contamination of theruthenium distillate with an acid and the explosion danger associatedwith the highly unstable nature of ruthenium tetraoxide.

Other methods for the separation and purification of ruthenium usingsolvent extraction and ion exchange methods have met with limitedsuccess and usually involve solvent extraction from a nitric acidsolution. In such solutions ruthenium occurs as a series ofnitrosylruthenium nitrate complexes that can be separated from thesolution by solvent extraction with, typically, long chain tertiaryamines. It is well known that ruthenium forms a very large number ofnitrosylruthenium complexes and that the stability of such complexes isgreater for ruthenium than for any other element. Thus, for example, inhydrochloric acid solution the nitrosylruthenium complex RuNOCl²⁻⁵ canbe formed. This complex is highly extractable, may be formedpreferentially, and allows for the separation of ruthenium from theother platinum group metals. This process, however, has its owndrawbacks, including 1) the available methods of making thenitrosylruthenium complex typically yield only 90–95% and 2) the otherplatinum group metals present tend to form complexes that exhibitsimilar behavior towards anionic solvent extractants.

The '442 patent goes on to address these issues to provide a process forthe extraction of ruthenium as a nitrosylruthenium complex with bothhigh yield and selectivity with respect to the other platinum groupmetals.

Yet another alternative process for the purification of ruthenium metalinvolved zone-refining. According to this process, a sample of impureruthenium metal is subjected to one or more heat treatments to form azone of molten ruthenium, surrounded by solid ruthenium, and move thismolten zone along the ruthenium sample and thereby segregate impuritiesfrom the ruthenium. Although this technique can produce very pureruthenium, ruthenium's relatively high melting point (approximately2280° C.) makes this process very energy intensive and requires morespecialized equipment to implement than the applicants' invention.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a new and improved method capable ofpurifying ruthenium sources to obtain high purity ruthenium without theneed for high temperature processing, expensive reagents, complex seriesof wet processes, or expensive equipment required to practice prior artprocesses. According to the present invention, a gas stream includingozone (O₃) is brought into contact with a ruthenium source in one ormore reaction vessels. The ozone reacts with the ruthenium present inthe ruthenium source to form ruthenium tetraoxide (RuO₄), a compoundthat is a gas at the reaction conditions. The ruthenium tetraoxide,along with unreacted ozone and the remainder of the gas stream is thenfed into a heated collection vessel where a major portion of the gaseousruthenium tetraoxide is reduced to form ruthenium dioxide (RuO₂)deposits within the collection vessel. The deposited ruthenium dioxideis then reduced, preferably with hydrogen, to produce the purifiedruthenium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a basic embodiment of an apparatus that may be usedto practice the disclosed method of ruthenium purification.

FIG. 2 illustrates an alternative embodiment of an apparatus that may beused to practice the disclosed method of ruthenium purification thatincludes a hydrogen source.

FIG. 3 illustrates an alternative embodiment of an apparatus that may beused to practice the disclosed method of ruthenium purification thatincludes only a single reaction vessel.

FIG. 4 a illustrates a possible configuration for a reaction vessel inwhich a series of perforated plates are provided for the support of theruthenium source in the ozone-containing gas stream.

FIG. 4 b illustrates a possible configuration for a reaction vessel inwhich the ruthenium source is supported within a fluidized bed by theflow of the ozone-containing gas stream.

FIG. 5 illustrates a basic series of process steps that could be used toetch a ruthenium layer during semiconductor processing.

FIG. 6 illustrates a possible configuration for a collection vessel thatprovides a series of collection surfaces upon which the RuO₂ would bedeposited and subsequently reduced to form Ru.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a new and improved method capable ofpurifying ruthenium sources to obtain high purity ruthenium without theneed for high temperature processing, expensive reagents, complex seriesof wet processes, or expensive equipment required to practice prior artprocesses. According to the present invention, a gas stream includingozone (O₃) is brought into contact with a ruthenium source in one ormore reaction vessels. The ozone reacts with the ruthenium present inthe ruthenium source, likely according to the reaction (1), to formruthenium tetraoxide (RuO₄) a compound that is a gas at the reactionconditions.Ru+mO₃→RuO_(4+nO) ₂  (1)

In its crystal form, ruthenium (VIII) oxide (RuO₄), is a golden yellow,highly volatile, solid at room temperature. It has a melting point of25.4 degrees C. and a boiling point of 40 degrees C. It is sparinglysoluble in water (2% w/v at 20 degrees C.) and, although freely solublein carbon tetrachloride, can react violently with other organic solventssuch as ether, alcohol, benzene and pyridine. Ruthenium tetraoxide is astrong oxidizing agent that also reacts with many other organiccompounds like olefins, sulfides, primary and secondary alcohols, andaldehydes, and will also degrade benzene rings.

The ruthenium tetraoxide, along with unreacted ozone and other gases isthen fed from one or more reaction vessels into a heated collectionvessel, the conditions in the collection vessel being sufficient toconvert a major portion of the gaseous ruthenium tetraoxide intoruthenium dioxide (RuO₂) according to the reaction (2) below:RuO₄→O₂+RuO₂  (2)

The ruthenium dioxide deposits on the walls of the collection vesseland/or collection surfaces or substrates disposed within the collectionvessel. The collected ruthenium dioxide is then reduced to produce thepurified ruthenium. When using hydrogen gas as the reducing agent, thereduction proceeds according to the reaction (3) below:RuO₂+2H₂→Ru+2H₂O  (3)

The morphology of both the ruthenium dioxide and the resulting rutheniumwas related to the temperature of the collection vessel. The collectionvessel employed by the applicants was surrounded by a single zonefurnace that resulted in a non-uniform temperature profile along thelength of the collection vessel. The furnace temperature set point wasachieved near the midpoint of the collection vessel, with thetemperature decreasing towards both ends of the collection vessel. Thepredominant morphology of the purified ruthenium deposited in thecollection vessel was crystalline needles, but a finer grainedmirror-like region was also observed toward the inlet side of thecollection vessel. If desired, a collection vessel with a more uniformprofile could be utilized to produce deposits comprising essentially asingle morphology, either crystalline needles or a mirror-like layer.This crystallographic selectivity could also be employed to coatselected substrates, e.g. wafers or metallic substrates, with either thecrystalline needles or a mirror-like layer of ruthenium dioxide or,after reduction, ruthenium.

Similarly, a collection vessel could be provided with a series of heatedcollection surfaces (201) that could be more easily removed from thecollection vessel for recovery of the ruthenium as depicted in FIG. 6.Depending on the construction, the walls of the collection vessel itself(202) could be kept at or near room temperature to reduce the potentialfor RuO₂ deposition. Alternatively, a cooling fluid could be introducedinto optional shell (203) to cool the walls of the collection vesselbelow room temperature or simply to control the heating resulting fromthe proximity of the collection surfaces (201). For coating a substrate,a configuration similar to that shown in FIG. 6 could be utilized withthe collection surfaces (201) serving as platens or chucks that willboth hold and heat the substrates to be coated. For the production ofruthenium, however, the crystalline needle morphology is preferred asthe crystals may be gently removed from the walls of the collectionvessel to collect the desired ruthenium product.

EXAMPLE 1

Using the apparatus generally depicted in FIG. 1, 487.1 g of rutheniummetal sponge was charged in the first reaction vessel (2) and 499.6 g ofruthenium metal sponge was charged in the second reaction vessel (3).The ozone generator (1) supplied a mixture of ozone (11.5%) and oxygento the first reaction vessel at a rate of 3 liters/min. The observedreaction efficiency was just under 4%, assuming that reaction (4) is theprimary reaction, resulting in the reaction of 2.5 g ofRu+2O₃→RuO₄+O₂  (4)ruthenium in the first reaction vessel and 1.0 g of ruthenium in thesecond reaction vessel. The ozone reacted with the ruthenium metalsponge at room temperature and pressure to form ruthenium tetraoxidewhich was visible in the reaction vessels (1, 2) as a pale green gas.The ruthenium tetraoxide, together with the unreacted ozone and oxygen,was then fed into the collection vessel (4) which comprised a 32″×2⅜″i.d. (approximately 813 mm×60 mm) glass tube with a wall temperature ofapproximately 450° C. After a reaction time of 160 minutes, thecollection vessel exhibited an accumulation of ruthenium dioxide, thepredominant morphology being crystalline needles. The system was thenpurged with nitrogen gas to remove the residual oxygen and ozone.Hydrogen gas was then fed into the collection vessel, again with a walltemperature of approximately 450° C. at the midpoint, to reduce thedeposited ruthenium dioxide to obtain ruthenium metal. Clearly, if therewas a need to obtain ruthenium dioxide, the deposits within thecollection vessel could simply be removed prior to the reductiontreatment. The collection vessel was then disassembled and the rutheniummetal collected. An analysis of the composition of the startingruthenium metal sponge and the recovered ruthenium metal is shown inTable 1 below.

TABLE 1 Trace Starting After Purification Metal (ppm) (ppm) Ni 2 0.3 Cu2 0.1 Fe 88 2.0 Al <10 0.3 Ca 15 0.2 Si 76 10 Mg 8 0.1 Cr 4.1 0.6

Among the advantages provided by the present invention is the highpurity achieved by an essentially “dry” process that does not requiretemperatures above 500° C. and eliminates the need for expensivereagents or resins and does not require highly specialized equipment.Another advantage is that the concentration of ruthenium tetraoxide isrelatively low and its residence time in the apparatus is relativelyshort (as it flows from the reactor into the collection vessel) so therisk of explosion is minimized. The applicants also believe that therelatively limited residence time also reduces the formation ofundesirable byproducts within the apparatus and/or on the rutheniumsource.

The reaction rates obtained with experimental apparatus utilized by theapplicants (FIG. 1) were limited by the physical limitations of theequipment. The applicants fully anticipate that both major and minorchanges in the apparatus and process conditions would improve, perhapssubstantially, the system performance without compromising theadvantages of the present invention. In particular, changes thatresulted in an increase the degree of contact between the ozone and theruthenium source during the residence time would be beneficial. Based onlimited experimental observation, the applicants believe that theruthenium-ozone reaction rate does not exhibit a linear relationshipwith increasing ozone concentration. Indeed, it appears that thereaction efficiency improves dramatically above a threshold ozoneconcentration of between 10% and 11% for reactants near roomtemperature.

One configuration that would achieve this improvement involves flowingthe ozone/oxygen mixture through a large chamber (150) containing aseries of trays (151) loaded with a thin layer of the ruthenium sourceas depicted in FIG. 4 a. Another configuration that would achieve thisimprovement would be a fluidized-bed chamber (170) in which theparticles of the ruthenium source (171) are suspended and agitated bythe oxygen/ozone flow as depicted in FIG. 4 b. Yet another configurationwould employ mechanical means to induce turbulent gas flow and/oragitate the ruthenium source to promote more uniform mixing between thegas and solid phases. The applicants also anticipate that adjustments tothe oxygen/ozone mixture composition, the gas flow rate, and thereaction temperature may provide further improvements to the efficiencyof the basic process for ruthenium purification.

In addition to the purification of ruthenium and ruthenium compounds,the basic chemistry embodied in the present invention has practicalapplication as an etch process for semiconductor, circuit board, orother instances in which a thin ruthenium film is to be etched. Comparedto other metal etching processes, the low temperature and relativelyhigh pressures (near ambient pressure and temperature) at which ozonecould be used to etch ruthenium layers eliminate the need for expensiveprocess gases, vacuum chambers and their associated load locks andvacuum pumps, and reduce the overall power consumption associated withthe etch process. Any ozone necessary for the process could be producedon site, thereby, avoiding the risks associated with the transportation,storage, and use of high pressure gas cylinders. Similarly, anyunreacted ozone could be removed from the exhaust stream by knowncatalytic or temperature treatments, thereby reducing any environmentalimpact of the present process when compared with some of the prior artetch chemistries. The advantages provided by the on-site production ofthe ozone are, of course, equally applicable to the disclosed processfor the purification of ruthenium. As envisioned by the applicants, atypical etch process would start with a semiconductor wafer having asits surface layer ruthenium metal. The wafer would be patterned withsome barrier material, such as photoresist, and then contacted with anozone-containing gas stream to oxidize the exposed ruthenium metal toproduce the volatile ruthenium tetraoxide RuO₄ that is then removed fromthe etch chamber. The basic process flow for such a process is providedin FIG. 5.

1. A method for forming a high purity ruthenium pattern on asemiconductor substrate comprising the steps of: placing a rutheniumsource in a first container; feeding an ozone-containing gas stream intothe first container; forming a reaction gas stream comprising ozone,oxygen, and ruthenium tetraoxide; feeding the reaction gas into acollection vessel containing the semiconductor substrate; reducingruthenium tetraoxide from the reaction gas on a major surface of thesemiconductor substrate to form a ruthenium dioxide layer on the majorsurface; purging the collection vessel to remove essentially allremaining reaction gas; and reducing the ruthenium dioxide layer toobtain a high purity ruthenium layer on the semiconductor substrate. 2.The method for forming a high purity ruthenium pattern on asemiconductor substrate according to claim 1, wherein hydrogen gas isused in the step of reducing the ruthenium dioxide.
 3. The method forforming a high purity ruthenium pattern on a semiconductor substrateaccording to claim 2, wherein the major surface of the semiconductorwafer is maintained at a temperature of at least about 110° C.
 4. Themethod for forming a high purity ruthenium pattern on a semiconductorsubstrate according to claim 2, wherein the major surface of thesemiconductor substrate is maintained at a temperature sufficient toboth reduce the ruthenium tetraoxide and decompose any residual ozone inthe reaction gas.
 5. The method for forming a high purity rutheniumpattern on a semiconductor substrate according to claim 4, wherein themajor surface of the semiconductor substrate is maintained at atemperature of at least about 450° C.
 6. The method for forming a highpurity ruthenium pattern on a semiconductor substrate according to claim2, wherein the high purity ruthenium layer is characterized by apredominate ruthenium crystalline morphology.
 7. The method for forminga high purity ruthenium pattern on a semiconductor substrate accordingto claim 1, wherein the high purity ruthenium pattern contains rutheniumhaving a purity of at least 99.99%.
 8. The method for forming a highpurity ruthenium pattern on a semiconductor substrate according to claim1, further comprising: removing the semiconductor substrate from thecollection vessel; forming an etching pattern on the high purityruthenium layer that exposes regions of the high purity ruthenium layer;etching the ruthenium layer to remove the exposed portions of the highpurity ruthenium layer and thereby form a high purity ruthenium pattern.9. The method for forming a high purity ruthenium pattern on asemiconductor substrate according to claim 8, wherein: etching theruthenium layer includes contacting the exposed portions of theruthenium layer with ozone.
 10. The method for forming a high purityruthenium pattern on a semiconductor substrate according to claim 9,wherein: the ozone is present in a fluid selected from a groupconsisting of liquid, gas and plasma.
 11. The method for forming a highpurity ruthenium pattern on a semiconductor substrate according to claim10, wherein: the ozone is the primary etchant present in the fluid. 12.The method for forming a high purity ruthenium pattern on asemiconductor substrate according to claim 8, further comprising:removing the etching pattern from a remaining portion of the rutheniumlayer after removing the exposed portions of the ruthenium layer toozone.